The recent drive for higher integration and operating speeds in CPUs, driver ICs, memories and other LSIs used in electronic equipment such as personal computers, digital video disks and mobile phones has increased the power consumption. Concomitantly the heat release has increased, which can cause malfunction of electronic equipment or damages to electronic components. It becomes very important to address the heat dissipation.
In prior art electronic equipment, heat sinks in the form of plates made of brass and other high thermal conductivity metals are used in order to suppress a temperature rise of electronic components during operation. The heat sinks conduct the heat generated by the electronic components and release the heat from their surface by means of a temperature difference from the ambient air.
For effective heat transfer from electronic components to heat sinks, the heat sink must be placed in intimate contact with the electronic component. Because of height differences among various electronic components and tolerance in the assembly process, a flexible heat conductive sheet or a heat conductive grease is interposed between the electronic component and the heat sink so that the heat transfer from the electronic component to the heat sink is established through the heat conductive sheet or heat conductive grease. The heat conductive sheet used is typically a heat conductive sheet made of heat conductive silicone rubber or the like (i.e., heat conductive silicone rubber sheet), and the heat conductive grease used is typically a heat conductive silicone grease.
The conventionally used heat conductive silicone rubber sheet, however, has a substantial contact thermal resistance at the interface with an electronic component, which imposes a certain limit to the heat transfer performance. This is a serious problem when electronic components of high-frequency drive with substantial heat generation such as CPUs are to be cooled. It is desired to reduce the interfacial contact thermal resistance.
On the other hand, heat conductive grease has a substantially negligible level of interfacial contact thermal resistance because of its liquid-like nature and exerts good heat transfer performance, but suffers from the problems that a special equipment such as dispenser is needed and its recovery entails an inefficient operation.
To overcome some of the above-described problems, a phase change heat dissipating sheet (simply phase change sheet) was proposed which is a solid sheet at normal room temperature, but softens by the heat generated by a electronic component during operation, to reduce the interfacial contact thermal resistance to a negligible level. The following phase change sheets were proposed in the prior art. U.S. Pat. No. 4,466,483 discloses a non-metallic sheet having phase change wax layers formed on opposite surfaces thereof. Also, U.S. Pat. No. 5,904,796 discloses a metal foil having a phase change paraffin or petroleum jelly formed on one surface thereof and an adhesive formed on the other surface thereof. JP-A 2000-509209 discloses a phase change sheet characterized by comprising an acrylic pressure-sensitive adhesive, a wax and a heat conductive filler, with an intermediate layer of network structure, film or the like being excluded.
On the other hand, the heat dissipating sheet is, on use, interposed between a heat dissipating member such as a heat sink and a heat generating electronic component such as CPU. In the recent industry, a situation has arisen where the process of producing personal computer components and the process of assembling such components to complete personal computers are separately performed at different locations over the world. It thus becomes a routine to attach a heat dissipating sheet to a heat sink at one location and transport the assembly to another location where it is mounted to CPU.
When the heat dissipating sheet-attached heat sink assembly is to be inspected for quality, the assembly must be mounted to a model CPU and subjected to a burn-in test or thermal cycling test under the same state as actual mounting. In the test, the phase change sheet works well because it melts at a temperature above the melting point and comes in close contact with the heat sink and the CPU. However, the phase change sheet can fail when the heat sink is separated from the CPU after the test. Even the electronic component and the heat sink can also fail because of strong adhesive bonds. In the event of failure of the phase change sheet, not only a new phase change sheet must be attached again, but a test must be done again to inspect whether the new phase change sheet meets the desired quality, which is unreasonable. After assembly to a personal computer, a burn-in test or thermal cycling test is sometimes done, giving rise to the same problem.
Moreover, U.S. Pat. No. 5,550,326 disclose a heat dissipator or heat dissipating structure in which an adhesive heat conductive pad layer is formed on one surface of a metal foil and which is used such that the surface of the adhesive heat conductive pad layer is in close contact with the electronic component side and the surface of the metal foil is in close contact with a heat dissipating member. This structure has the problem that since the adhesive heat conductive pad is in close contact with the electronic component, an attempt to peel the adhesive heat conductive pad from the electronic component can result in failure of the adhesive heat conductive pad and even failure of the electronic component.
The present invention intends to improve the above-discussed situation and its object is to provide a heat dissipating structure for a heat generating electronic component, which can effectively conduct the heat generated by the electronic component to a heat dissipating member such as a heat sink, and which even after a burn-in test or thermal cycling test to be performed for quality inspection, allows the electronic component and the heat dissipating member to be separated without structural failure of a heat dissipating sheet or failure of the electronic component, as well as a heat dissipating sheet used therein.